old shutter are the shiznat.. But allow me to offer a couple corrections.
the way ROF works with electric guns, is that the higher the ROF is, the more "windows" you have to pull the trigger, and for the gun to recognize the trigger pull and fire.
Barrel length doesnt really matter at all, as long as its between 12 and 16 inches. 14 is recommended, but if you end up playing speedball, depending on your position or playing style, you'll want a longer barrel.
There are all sort of low-end guns out right now, i would suggest going to a store and checking them all out and see which one you feel the most comfortable with. However, if you want a foolproof gun, get an automag, you will almost never have any trouble with it, i can almost guarantee it.
if you want to shell out massive cash, get a smart parts shocker, I had an Angel Speed, which was the highest end gun at the time when i stopped playing, but once the shocker came out, it shred it to pieces, though the solenoid at the time was having issues blwing out... so i'd read up on that. a good forum to check out is www.pbnation.com
hope that was helpful